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Heatstroke: The reality doesn’t fit with the perception

One of the more interesting ways in which we can study physiology (especially during exercise), is to observe it when it fails. Take for example the Calvin and Hobbes equivalent of “failure physiology”:

Calvin (the young boy, for those who haven’t discovered Calvin and Hobbes) asks his father a seemingly simple question, and gets an absurd answer. Yet incredibly, this is how exercise scientists have approached certain problems for many years – fatigue and temperature is the most obvious of them! So we study what happens at failure (exhaustion) and then infer the cause backwards from there! For example, when studying fatigue, many exercise physiology studies make runners or cyclists exercise at a fixed workload until they are absolutely exhausted and then measure things at the point at which they stop, assuming them to be the cause.

“We happened to notice that the runners all stopped when they had low glycogen levels/high body temperature/high lactate levels. Therefore, they stopped because of it!” is the logic applied (we covered this in our fatigue series earlier this year).

When the body overheats (i.e. heatstroke), it’s the classic illustration of this “load limit” concept for humans during exercise. We know from these “run until you drop” studies that humans almost always stop exercise when their body temperature hits 40 degrees. This has been called the “critical limiting temperature” (Calvin’s dad would be proud), and the theory was that muscle activation fails at this point. It was subsequently shown that if you allow the athlete to choose their own speed (like you do in every exercise situation), they slow down long before they overheat – we called this Anticipatory Regulation of exercise by the brain.

The problem with this theory – physiology does fail

This anticipatory theory, and even the “load-limit” theory, do pose some problems for us, however, because they imply that humans will never exercise so hard that they drive their body temperature to even higher levels. Recall that heatstroke is defined as an increase in body temperature above 41 degrees celsius (or 104F), which would be very difficult to explain if your brain “fails” at 40 degrees, or regulates you so that you never get there.

Of course, you may argue that some people can “over-ride” this physiological regulation, but I would argue that this is akin to suggesting that someone can commit suicide by holding their breath. It doesn’t matter how badly you want it, but physiology always wins the day! I will concede that some people have exceptional “mental toughness” and approach the limit much more than others, but no one breaks right through it. And now, consider that heatstroke very rarely happens in the elite athletes, who would surely be the ones most likely to have this “mental override” capacity. So it seems an oversimplification to say that heatstroke happens because people “push themselves” too hard in the heat.

Similarly, when you actually look at cases of heatstroke, something even more intriguing jumps out at you – heatstroke has almost never been reported on hot days! Look at the following table, which shows 18 documented cases of heatstroke.

Hopefully, you’ll be struck by the relatively low temperatures at which many of them have occurred. There are races, for example (cases 3 to 9), where temperatures are below 20 degrees, sometimes only 4 degrees celsius! That should set off an alarm in your mind about how heatstroke occurs – is it simply a case of exercising in the heat until you overheat? It seems more complex than this. And what I’ll do in our next post is look specifically at two or three of these cases and show how it’s physiologically “impossible” to explain these cases according the classic physiology that you run yourself into trouble. Basically, it’s impossible to run fast enough to develop heatstroke unless you’re very heavy and run very fast…(but more on that in the next post of this short heatstroke series)

Diagnosing heatstroke

These 18 cases are among the ONLY documented cases of heatstroke, which is itself incredible. You probably think that heatstroke is very common, because every time there’s a race in vaguely hot conditions, you’re warned to drink lots of fluid and take every precaution to avoid heatstroke (especially in the USA, I must say. That’s a fact). But incredibly, exercise science and sports medicine often even fail to measure that load limit before they attribute just about any “failure” in the heat to heatstroke! So, returning again to the definition (a body temperature that exceeds 41 degrees celsius), you’d be amazed at how often heatstroke is “diagnosed” without ever measuring the body temperature. Instead, the default response to a case where an athlete can’t exercise and it happens to be a relatively warm day outside is “heatstroke” regardless of what the person’s body temperature is.

So I must stress that there is a big difference between heatstroke and what one might term “exercise intolerance due to feeling terrible in unaccustomed heat,” which is what I think accounts for most of the problems people experience during running. And in our discussion of the Chicago Marathon, it was mildly amusing to read some responses that were indignant at my suggestion that some people expected marathons to be “easy”, whereas others (those in the more demanding climates around the world and thus used to the heat) have a completely different view of running in different conditions. In some places, a marathon at 24 degrees with moderate humidity is a pleasure. For others, it’s a cauldron of heat and danger…!

The physiological abnormality of exercise

Returning to the heat issue, what may surprise you is that during exercise, almost regardless of the air temperature, humidity and windspeed, your body temperature will regularly hit about 39 degrees celsius, with no ill effects whatsoever – it’s a controlled “hyperthermia”, and you’re halfway to heat stroke without ever even realising it! It’s actually amazing to consider how exercise makes the “abnormal” feel normal. Take a physiological snapshot of yourself during a 10-mile tempo run and your heart rate is 175 beats per minute, your breathing rate 54 breaths per minute, your body temperature is 39 degrees celsius. A doctor presented with those statistics would likely admit you to an ICU, yet you feel absolutely perfect during exercise!

The point is that the perceptions you have of your own physiology are strongly influenced by set-points. That is, your brain can adjust what is “acceptable”, the set-points, to create a context to interpret incoming physiological signals. Think, for example, of when you have a fever – you are shivering even though your body temperature is 39 degrees celsius and you’ve overheating. That’s because your body’s set-point has been reset and you are kicking in heat GAIN mechanisms even though you’re hot – how you FEEL is not necessarily the same as how you are ARE. A similar concept applies to exercise.

And the point of all this is to introduce the issue of heatstroke to you. Your body is a remarkably designed machine, capable of losing far more heat than you might realise. Yet it CHOOSES to allow you to gain heat and you become “hyperthermic” during exercise even on cold days. Where this is leading me is to the next post on heat stroke, where I’ll look at the physiology of heat gain and heat loss and explain how actually running yourself into overheating is a pretty tall order. Instead, there is something else that accounts for heat stroke, and we’ll put forward a few suggestions about what it might be.

Did you know?

We published The Runner's Body in May 2009. With an average 4.4/5 stars on Amazon.com, it has been receiving positive reviews from runners and non-runners alike. Available for the Kindle and also in paperback.